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The Parasitic Vine That Slowly Sucks the Life Out of Wasps

Galls with a parasitic vine wrapped around themBrandon Martin / Rice University

Every year, Scott Egan and his students crisscross the country looking for odd, round things called galls that grow on plants. They are, essentially, plant tumors. Once you start looking, they’re everywhere. Egan stops at county parks, cemeteries, church parking lots, even the side of the road. It was on one of these trips, to Florida, that they picked up a gall unlike any Egan had seen in 15 years of studying plant tumors.

Well, actually, he didn’t even realize it at the time. When a student brought him the unusual-looking gall—it had a vine wrapped around it—Egan dismissed it as a seed or fruit of the vine. Luckily, someone had the presence of mind to investigate further. They broke it apart and at the center was a single adult wasp, dead and mummified.

To Egan, an evolutionary biologist at Rice University and a connoisseur of galls, the wasp itself was not that surprising—at least once he accepted that the pea-sized brown nodule truly was a gall. Such tumors are often caused by parasitic wasps called, appropriately, gall wasps. They lay eggs inside of plants. Once hatched, the larva tricks the plant into growing a swollen, nutrient-rich gall, providing itself with food and shelter. Once it’s taken and taken and taken from the plant, the fully grown wasp bores out of the gall and begins its adult life.

Scott Egan holds up a clump of galls he found in the field. (Scott Egan and Glen Hood)

What really intrigued Egan was the vine attached to the gall. The Florida oak trees where his team found this unusual gall were covered in a yellow tangle of love vine, a parasitic plant. Love vines send their suction-cup roots all over the oak, sucking out nutrients. These poor Florida oaks were being doubly parasitized—by the gall wasps and the love vine at the same time. When the two parasites met, something weird happened. The love vine hijacked the gall wasp’s hijacking of the oak tree, siphoning food out of the nutrient-dense gall. Ultimately, the wasp died.

“The biter is bit!” says Graham Stone, a biologist at the University of Edinburgh and longtime researcher of insect galls. He had never seen anything like this, either. “It’s really good fun,” he says of the find, though he admitted, it’s also “pretty freaky.”

There is a elegance to this system. Galls are, as Stone puts it, “a lump of yumminess.” But animals normally avoid them because the tough outer covering is full of tannins, bitter chemicals that plants use to ward off predators. (So full of tannins are galls, in fact, they have long been used to “tan” leather.) But bitter tannins don’t bother plants like the love vine. Gall wasps so cleverly co-opted the oak tree’s chemical-defense system to protect themselves against animals; they never knew they also had to watch out for plants.

“I think this is a fascinating new interaction … a new facet of all the possible interactions galls can have,” Karsten Schönrogge, an ecologist at the Centre for Ecology & Hydrology, wrote of the discovery in an email.

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Galls do have plenty of bizarre interactions. Some gall wasps, for example, induce galls that produce a sweet nectar-like substance to entice ants. These ants guard the gall and the soft, developing larva inside from yet other parasitoid wasps that specialize in laying eggs inside other wasp larvae. If the ants leave, the parasitic wasps attack the gall. The gall wasps “are using the resources of the tree to pay for a mercenary army to protect itself,” Stone says.

Since the first love-vine-and-gall-wasp discovery, Egan’s team has gone back to the roadside site in Florida and found a couple dozen more. The site, he emphasizes, is nothing exotic. “We’re finding these things right here in the U.S., sometimes right next to major cities. This site isn’t that far from Miami,” he says. Still more new things must be hiding in plain sight, right next to the roads we speed past everyday.